The present invention relates to an optoelectronic device that is used in fiber-optic communications. More specifically, the present invention relates to an improvement in the construction of a p-i-n photodetector to enhance its response time over an operating wavelength range from 700-1600 nanometers (run).
The development of the Internet and other datacom networks has created an ever-increasing need for high rates of data transmission. Optical links, with their ultrawide bandwidth and low-distortion fiber transmission, are increasingly favored over traditional copper-wire approaches. Optical links operate at one of the following wavelengths: 780, 850, 1310, and 1550 nm, with 1310 nm and 1550 nm used primarily for long-haul applications, where their ability to propagate distortion-free in single-mode optical fiber is critical. For short-haul applications, which include workgroup LANs (local area networks) and campus backbones, the number of components implemented can be considerably higher, causing their costs to become a key factor. Short-haul networks are often designed to operate at the shorter 780 and 850 run wavelengths, where directly-modulated lasers can be manufactured less expensively using VCSEL (vertical-cavity surface-emitting laser) technology. Multi-mode, 62.5 micrometer (.mu.m) diameter fiber is the fiber of choice for these systems. This large a core fiber means that equally large-area detectors are required. Both Si- and GaAs-based photodetectors are available for this application, provided the modulation rate is below 1.25 Gbit/s (Gigabit Ethernet). Above 1.25 Gbit/s, GaAs detectors are preferred.
With 1.25 Gbit/s systems now being implemented, network providers have moved towards development of a 10 Gbit/s link that also uses 62.5-.mu.tm multi-mode fiber. This effort is at the research/development level and high-speed diagnostics are now needed for their characterization. In general, components that can operate at 10 Gbit/s need only have an 8 GHz bandwidth. One of the components that has proved difficult to develop is an 8-GHz photodetector that is sensitive to 780 nm and 850 nm light. GaAs-based detectors cannot satisfy both specifications. The limitation with GaAs stems from its low absorption coefficient at the shorter wavelengths. Indeed, GaAs-based 8 GHz detectors have been fabricated, provided the active region is no greater than 2 .mu.m in thickness. This thickness assures that all the optically-generated electrons and holes sweep out sufficiently fast to achieve 8 GHz bandwidth. However, to obtain near unity quantum efficiency at 850 nm from a GaAs detector would require the active layer be &gt;4 .mu.m. This is feasible, by going double-pass through the 2-.mu.m region, but is prohibitively expensive to manufacture and package. The situation improves somewhat for light at 780 nm. However, single-pass illumination through 2 .mu.m would still result in less than unity quantum efficiency.
The ideal semiconductor for this application is In.sub.0.53 Ga.sub.0.47 As grown lattice matched on semi-insulating InP (InP:Fe). In.sub.0.53 Ga.sub.0.47 As has a lower bandgap than GaAs and can provide equivalent absorption at 850 nm with a quarter of the thickness. The 4 .mu.m thickness required for full absorption in GaAs reduces to 1 .mu.m in In.sub.0.53 Ga.sub.0.47 As. At this thickness, detector bandwidths can exceed 20 GHz. If a 2 .mu.m In.sub.0.53 Ga.sub.0.47 As layer is used, we can obtain the needed 8 GHz and also have strong absorption out to 1550 nm. In.sub.0.53 Ga.sub.0.47 As-based p-i-n photodiodes have been available for some time for use at 1300 nm and 1550 nm. These photodiodes are heterostructures, consisting of an undoped, relatively thick In.sub.0.53 Ga.sub.0.47 As active region sandwiched between thin, heavily-doped p and n In.sub.0.52 Al.sub.0.48 As regions. These are most often back-side (substrate-side) illuminated detectors. The light propagates through both the substrate and transparent In.sub.0.52 Al.sub.0.48 As n-doped layer before being absorbed by the active In.sub.0.53 Ga.sub.0.47 As layer. The cut-off wavelength for back-side illumination is determined by the absorption edge of the InP and is 900 nm. For detection at 780 nm or 850 nm, a front-side design is needed, and requires that the p-doped top layer be transparent to allow passage of the light. A front-side illuminated p-i-n photodiode based on In.sub.0.53 Ga.sub.0.47 As could, in principal, have quantum-limited sensitivity at 780 nm or 850 nm and also have a bandwidth of 8 GHz. What prevents this bandwidth from being realized is the sheet resistance of the transparent p-contact.
In addition to the sweep-out time discussed earlier, the response of a photodiode can be limited by its RC time constant. The RC time constant is the parasitic response of the photodiode and is the product of the diode's series resistance, R and capacitance, C. For a photodiode to collect all the light from a 62.5-.mu.m core fiber (the most common fiber size for short-haul applications), it must have a diameter of at least 62.5 .mu.m. Taking the active layer thickness to be 2 .mu.m, yields a capacitance for a photodiode of .about.0.2 pF. For the case of a back-sided illuminated detector, the total series resistance can range from 20-50 .OMEGA., depending on the contribution from contact resistance and the resistance of the n-doped layer. For this detector, we must rely on lateral conduction through the n-doped layer to transport charge, and so the sheet resistance value for the n-layer is critical. The resistivity of a layer doped with shallow donors can be reduced by increasing the dopant concentration. The most widely used shallow donor for n-type contacts is tin (Sn). Sn can be doped to a level of 10.sup.20 cm.sup.-3 before diffusion becomes a problem. At this concentration, the resistance for the n-doped layer is .about.20 .OMEGA., for a 700-nm thickness. Note that this layer, though relatively thick, is transparent to 1300 nm and 1550 nm light. At the opposing contact is the p-doped layer. For a back-side illuminated detector this contact can be covered with a thin metal film on its outer surface to reduce its sheet resistance to &lt;1 .OMEGA.. If this photodiode were limited only by its RC parasitics (i.e. no sweep-out limitations), it would have a 10 picosecond (ps) response. In an typical back-side illuminated detector with a 2-.mu.m active layer, the RC time constant is faster than the charge sweep out time (.about.30 ps). Assuming Gaussian pulse profiles, the combined contribution from the two time constants is (10.sup.2 +30.sup.2).sup.1/2 =32 ps, which corresponds to .about.8 GHz bandwidth.
The situation changes for a front-side illuminated photodiode. For this geometry, the p-doped In.sub.0.52 Al.sub.0.48 As contact can no longer have a metal top coating The detector must rely on lateral conduction from both the n- and p-doped layers. To hold optical losses to .ltoreq.20% at 850 nm also requires the thickness of the p-layer be .ltoreq.400 nm. This challenge is further complicated by the fact that beryllium (Be) and zinc (Zn), the industry's standard p-dopants, cannot be doped to the same 10.sup.20 cm.sup.-3 concentration as done with Sn in the n-doped layer. This is because Be and Zn have a much higher diffusion coefficient than Sn. Above 5.times.10.sup.18 cm.sup.-3, Be, for example, begins to diffuse into neighboring regions moving most rapidly along defect channels. This causes Be to contaminate the undoped i-region of our p-i-n photodiode and greatly increases its dark current, or worse, shorts the diode. If we limit our Be concentration to a safe level (.ltoreq.5.times.10.sup.18 cm.sup.-3), where Be diffusion is minimal, the resistance for the p-doped layer could be as high as 50.OMEGA.. The bandwidth of this front-side detector degrades from 8 GHz to &lt;5 GHz.